1. Field of the Invention
The present invention relates to an apparatus for variable length encoding of image data and transmission encoding in accordance with a transmission path and, more particularly, to a multiplexing process of variable length encoding data and transmission synchronous information.
2. Related Background Art
In recent years, a digital processing system according to the MPEG (Moving Picture Experts Group) 2 standard [ISO/IEC (International Organization for Standardization/International Electrotechnical Commission) 13818-1 to 13818-3] is becoming a standard of a video transmitting system.
FIG. 1 is a block diagram showing a construction on the transmitting side of a general digital transmitting apparatus according to ISO/IEC 13818-1 to 13818-3.
In FIG. 1, reference numeral 301 denotes a video encoder for compressing digital video data so as to conform with ISO/IEC 13818-2; 302 an audio encoder for compressing a digital audio signal so as to conform with ISO/IEC 13818-3; 303 a packetizer for packetizing a video elementary stream as an output of the video encoder in accordance with the PES (Packetized Elementary Stream) of ISO/IEC 13818-1; and 304 a packetizer for packetizing an audio elementary stream as an output of the audio encoder in accordance with the PES of ISO/IEC 13818-1.
Reference numeral 305 denotes a TS (Transport Stream) multiplexer for packetizing a video PES and an audio PES as outputs of the packetizers into transport stream packets in accordance with the TS (Transport Stream) of ISO/IEC 13818-1 and multiplexing them.
The transmitting operation of the digital transmitting apparatus constructed as mentioned above will now be described hereinbelow.
The video data and audio data are inputted to the video encoder 301 and audio encoder 302, respectively. In the video encoder 301, information whose correlation is high with respect to the space and time is checked and converted into data of a low redundancy in accordance with ISO/IEC 13818-2, thereby compressing an information amount.
In the audio encoder 302, information whose correlation is high with regard to the time is checked and converted into data of a low redundancy in accordance with ISO/IEC 13818-3, thereby compressing an information amount. A unit which can be solely decompressed in those compressed data trains is called an access unit (AU), data trains of the video AU and audio AU are called a video elementary stream (video ES) and an audio elementary stream (audio ES), respectively.
The video ES and audio ES are respectively inputted to the packetizers 303 and 304 and are packetized (PES) to variable length packets generally on a unit basis in which the access unit is set to a reference together with a stream ID indicative of an attribute of the ES, time stamp information showing a decoding time and a display time on the decoding side, and the like.
The TS multiplexer 305 receives the video PES and audio PES, converts into transport streams (TS), and outputs.
FIG. 2 is a block diagram showing a detailed construction of the TS multiplexer 305.
In FIG. 2, each of the video PES and audio PES is accumulated in each buffer and is packetized on a unit basis for transmitting data. This packet is called a transport stream packet (TS packet). In ISO/IEC 13818-1, the TS packet has a size of 188 bytes and includes a header including a sync byte which is necessary to reconstruct the TS packet on the decoding side, a PID (Packet Identification) showing the ID of the TS packet, and the like, an adaptation field in which annexed information which is necessary to transmit PCR (Program Clock Reference) showing a reference time to specify a receiving time on the decoding side and the like are described, and the other space area (payload) into which the video PES or audio PES is stored. On the other hand, synthetic annexed information of the TS such as definition of the PID or the like is formed as PSI (Program Specific Information), is stored into a buffer, and is packetized to a TS packet having the PID which has previously been defined.
As for the PCR, since it has to be outputted as a TS within a period of time of 100 msec, generally, the PID for PCR is defined by PSI separately from the packets to transmit the video PES and audio PES and is packetized as a TS packet constructed by only the packet header including the PID and the adaptation field.
Each TS packet is read out from each buffer at a proper timing on a TS packet unit basis and is outputted as TS.
According to the multiplexing method of data in the digital transmitting apparatus as mentioned above, there is a large limitation in an amount of data which is transmitted onto the transmitting path, the transmitting path cannot be effectively used, and a vain stuffing data always has to be multiplexed.
For example, when the video PES shown in FIG. 3 is converted into the TS packet, the adaptation field is inserted into the last TS packet and the vain stuffing byte (dummy data bytes for making a packet data length constant) have to be multiplexed.
Even in case of transmitting the independent TS packet for the transmission of PCR as shown in FIG. 4, the stuffing process is necessary in the TS packet.